Today I learnt something new.
I was playing "truth" (planned to play all versions actually *different arrangements!*) when I saw the word 「gliss」on the next page's song (Kaze no Mukou e/the intro). Actually I've often see this word on various piano sheets/books but never cared enough to look up for its meaning. When I finally did today :
"OH!"
*remembers that charming part of Sho's piano rendition of "Hatenai Sora"*
So I tried it out and... WHOA. Painful!
LOL. I realised this is the painful part of playing the piano.
Then I googled for a less painful way to play glissando on piano. And I found it here. *useful tips!*